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Why total drag formula include drag at zero lift?
Total drag  =  Do + DL
Why total drag include drag at zero lift if we are interested at drag when wing produce lift?
We cant assume that friciton and particularly pressure drag at zero lift AoA dont change when AoA increase.

I'm adding this separate answer to provide evidence that induced drag can be non-zero at zero lift.
First, I performed a simple aerodynamic analysis on a simple straight wing.  This has a symmetrical airfoil and no twist.  I analyzed it at zero angle of attack.  As expected, there is zero lift, zero drag, and pretty much nothing going on...
First, the lift distribution plot.  No lift anywhere (note the axis scale).

Next, the induced drag distribution plot.  Nothing to see here.

Here is a visualization of the flowfield.  Since this is a thin-surface method, we present $\Delta C_p$ instead of $C_p$.  Note the scale -- nothing is happening, it is all just equivalently zero.
This also depicts the wake lines.  Notice that the wake is perfectly flat.

Next, I analyzed the same wing, but with ten degrees of twist about the quarter chord (positive LE up).  Here is a polar plot of the induced drag.  This plot was generated by sweeping angle of attack from -10 to 10 in one degree increments.

The $CDt$ label refers to the Trefftz plane analysis -- the particular technique used to calcualte the drag for this case.
Notice that the bottom of the parabola never goes to zero.  The minimum drag is about .0025 and occurs at about -5 deg angle of attack.  This is induced drag.
Next, we'll take a look at the lift distribution for this wing.

Notice that since the tips were twisted leading edge up, with the wing at -5 degrees, the tips have positive lift -- and the middle of the wing has negative lift.  The combined sum of this is close to zero.
Induced drag is caused by changes in this lift distibution.  If you have an ugly distribution of lift, you'll have a lot of induced drag.  If you have a beautiful (elliptical) lift distribution, you'll have less induced drag.
Here is the drag distribution for this wing at zero total lift.

While there appears to be a section near mid-span that contributes zero drag (note, this would be a section producing zero lift), the rest of the wing contributes to the drag -- whether it is lifting or negative lifting.
Edit: There was some question about how the lift and drag distributions would vary with angle of attack.  Here are those charts.

As you can see, if you go to low enough angle of attack, the entire wing is producing negative lift -- and if you go to high enough angle of attack, the entire wing is producing positive lift.
The approximately zero lift case is the blue circles 5th from the bottom.

Here are the drag distribution charts for all angles of attack.  The blue circles are hard to see in this case because they're squeezed into the bottom and overlaid by many other results.
Interestingly, at high angle of attack, there are some cases wehre the center section produces a slight thrust -- this is a leading edge suction effect.
Here is the flowfield image for this solution.

You can see that this is very different from the zero-twist zero-alpha zero-lift zero-drag case.  Keep the scale in mind -- the other scale was essentially zero everywhere.
Finally, I've arranged the wing so you're looking at it from about the oncoming wind (from the front).  You can see that some of the wake is pushed above the wing and some of the wake is pushed below the wing.

There is clearly upwash and downwash going on -- the wake shape allows you to nearly 'see' the production of induced drag.

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There are (at least) four kinds of drag due to pressure.

Induced drag.  This is the drag associated with the process of creating lift.  It directly depends on the shape and magnitude of the lift distribution in the wake.  It is strongly a function of lift.

Form drag.  This is a pressure drag term associated with the thickness of the boundary layer on a body that is not a flat plate.  Although this is a pressure drag, it is a result of viscous processes and the boundary layer.  It is a weak function of lift (or angle of attack).

Separation drag.  This is a pressure drag associated with flow separating from a body.  Inviscid flow would stay attached, so this is also a viscous phenomena.  If the separation changes with lift, then this is a function of lift.  Separation around a protuberance (say an antennae, fastener, gap, etc) will likely not vary much with lift or alpha.  Separation due to stall is of course a strong function of lift or alpha.

Wave drag.  This is pressure drag associated with the presence of shock waves in high speed flow.  This will typically be broken into a wave drag due to volume and a wave drag due to lift.  The lift term is obviously a function of lift.

There are others, but they also start to become blurred...

Transonic drag rise.  This is mostly a result of shocks on/around the aircraft causing separation.  When you accelerate through the transonic region to pure supersonic flow, the shocks move off the body, the separation goes away, and you're back down to just the wave drag.

Trim drag, cooling drag, interference drag, etc.  These are generally made up of the earlier causes of drag, but are book-kept separately because it is useful in an engineering sense.

The only drag that is not a pressure drag is skin friction drag.  If you think about a small element of an aircraft's skin, it can experience a pressure force (normal to the skin), or a shear force (parallel to the skin).  The only source of a shear force is skin friction.  Everything else is a pressure force.
Often, for 2D airfoil data, the skin friction and form drag terms are summed together and are called 'profile drag'.
Low speed aircraft won't have 4,5.
Clean, well designed aircraft will work their hardest to avoid 3) as much as possible.
The only way to have zero induced drag at zero lift is for the entire lift distribution to be zero (every airfoil across the wing must have zero lift).  This is only true for a wing with no twist and that uses symmetric airfoils.
If a wing has camber and/or twist, the zero lift situation will be made of some parts of the wing lifting up -- and other parts lifting down.  The total lift is zero, but the lift distribution is not.  Consequently, this wing will have non-zero induced drag at zero lift.

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Has resultant pressure vector backward component at 2D airfoil in viscid flow?
When we integrate static pressure around 2D airfoil that produce  lift(viscid flow), has  resultant pressure vector backward component?
I think pressure vector will point slighlty back, so it has some drag component.
Some members here call induced drag as component of pressure vector point back, so why do we have pressure vector point back in 2D airfoil if induced drag must be zero?

Check this answer for some discussion on drag breakdown. (I now realize that was your question)
There is 2D pressure drag (not induced drag) that happens on an airfoil with finite thickness.  It is called form drag.  This is a viscous phenemona associated with the thickness of the boundary layer at the trailing edge of the airfoil.
There can also be separation drag and wave drag that are also pressure drag.
Induced drag is not the only source of pressure drag.
Some models of induced drag do involve analyzing the component of the lift vector (caused by pressure) that is tilted back.  Having that as an explanation of induced drag does not mean that is the only set of pressures in the drag direction.

Do you think it would be accurate to regard this question as a duplicate of the one you linked?

@RalphJ I don't know.  Clearly the first question raised more questions in the asker's mind and so they're asking for clarification.  I think I'm going to need to spend some time and do a proper description of form drag -- that one is a doozy.

@user207141 please don't ask new questions in comments. Instead ask a follow up question. Whilst part of your comments can be seen as requests for clarification, this thread is now getting to a level where it is becoming a chat and new questions are posted. Please use our chat to chat, use comments only for clarifications and ask new questions when the need arises.

@RobMcDonald I really appreciate the insights you are providing and by deleting all the comments here I don't want to discourage you in any way. Just trying to keep things organised. If there was anything important lost when I removed those comments, please give me ping and I will try and help out.

Let's start with an inviscid flow around a 2D airfoil which is easier. In this case the pressure:

decreases moving from the leading edge to the point of maximum thickness and;
increases afterward toward the trailing edge reaching again the ambient pressure (so called "pressure recovery").

These increase and decrease in pressure perfectly compensate for each other in the horizontal direction and their resultant in this direction (drag by definition) is therefore zero. These result is called D'Alambert paradox.
Viscosity is the property of a fluid to resist against deformations. On an aerodynamic body like an airfoil, this resistance typically happens inside the boundary layer: the several layers of fluid inside the boundary layer which are moving at different speeds slow down the aerodynamic body exactly due to resistance created by the viscosity between each of those layers. The net effect is a drag force which is termed "skin friction". Note that this drag has nothing to due with the pressure but only with the viscosity of the fluid; that's why it is normally called "friction" instead of "drag".
The boundary layer is anyway responsible for the creation of drag also via pressure. Due to its thickness, which increases from the leading edge to the trailing edge, the pressure recovery of the previous point 2) does not complete till the ambient pressure since the boundary layer is now in the way: 1) and 2) do not compensate anymore rather 1) is now always bigger than 2). But since the bigger under-pressure of 1) acts on the part of the airfoil which is mainly facing forward then its effect is to create a forward facing net force, i.e. a drag: this drag is termed "profile drag".
Increasing the angle of attack, the thickness of of the boundary layer increases as well and the pressure recovery on the back part of the airfoil becomes lower and lower i.e. the profile drag higher and higher. This increase in drag is more or less proportional to the square of the angle of attack. Increasing the angle of attack even more, at a certain point the boundary layer detaches completely from the  surface of the airfoil: the pressure recovery of 2) is now completely lost.
Induced drag is a pure mathematical 3D concept which is connected to the spanwise lift distribution on the wing. Obviously a 2D airfoil has no span and therefore the concept of induced drag is formally undefined for it.

Last paragraph makes one wonder what polar diagrams base their lift to drag ratios on, an infinite span?

If the object is moving but not lifting, it still will have drag.
It seems what is done in aviation is to separate pressure, as in dynamic pressure, or center of pressure, into "that which opposes forward motion" and "that which is used to create lift"

Coefficient of drag = C$_{di}$ + C$_{d0}$

This becomes the "thrust budget" of the aircraft.
Thrust can be gained from converting the potential energy of fuel or thermals (or somebody towing you) to the kinetic energy of propulsion or/and  utilizing gravity and altitude.
Thrust must equal the sum of all drags or airspeed is lost.

source: Stack Exchange

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Is Biot–Savart law physical in aerodynamics?
Biot–Savart law comes from electromagnetics theory.
Vorticity behind the wing(in the wake)  induce downward vertical velocity upstream of the wing, so effective AoA of the wing is reduced.
Is this really mechanism what happend in real fluid(air)?

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In subsonic flow, the differential equations are elliptic -- this means that everything influences everything.  In supersonic flow, the equations are hyperbolic -- which means that disturbances only move downstream, nothing downstream can influence anything upstream.
If you think of a simple 3D lifting wing model -- a single horseshoe vortex, then yes, the two trailing vortices are the source of induced drag and all the 3D effects.
A super-simple 2D lifting airfoil model is a single point vortex placed at c/4.  When extended to 3D, that would be the bound vortex of the wing -- the 'crossbar' between the trailing vortices.
The bound vortex causes an upwash ahead of the wing (and a downwash behind it).  The trailing vortices cause a downwash between them - and an upwash outside of them.  This region of downwash extends in front of the bound vortex - but is weaker than the bound vortex influence.
The trailing vortices cause the lift distribution to vary from its peak at the root to zero at the tips.

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No.
You start from wrong premises. Vorticity adds an upward vertical velocity ahead of the wing. This is called induced angle of attack. You cannot focus on the free vortices behind the wing without including the bound vortices within the wing. While the free vortices would indeed add a downward vertical velocity between their cores, the bound vortices are dominant for the induced velocities ahead of the wing and cause an overall increase in the angle of attack.

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Why I cant see this downward movement infront wing which reduce effective AoA, (I see only upwash)?
Circulation theory predict  downward airflow that reduce effective AoA,which cause induce drag.
To change effective AoA this downward airflow must be infront of wing, to "hit" wing with lower AoA, but not behind wing(in the wake).
In reality I only see upwash infront and downwash behind,so where is this flow?

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As the name suggests, circulation theory describes the lift as being produced by the airflow that "circles" around the 2D airfoil i.e. by an airflow going up in front of the airfoil, accelerating on its upper part, going down behind it and decelerating on the lower part.
If the freestream airflow's speed could be subtracted from the picture of your question, this circulation would be really nicely visible:

(Source: Daniel P. Raymer, Aircraft design: a conceptual approach)

my question refer to what we see in real word, so that mean wing dont fly inside its own downwash?

If you write a question about circulation theory you are going to get an answer about circulation theory. Rotating wings do fly inside their own wake/downwash.

There is also downwash along the entire surface of the bottom of the wing.
This (action/reaction) lift is what you have left when the upper wing stalls.
"Bottom lift" comes at a higher drag price.  More efficient "top lift" can be had at higher airspeeds and lower angles of attack.  Essentially, this is why Vbg is faster than Vmin sink.
Regarding "induced drag", one might posit that, since we define Total drag coefficient as Cdo + Cdi, why not call induced drag that which comes from increasing Angle of Attack relative to the freestream.  With airplanes, we generally do this to create lift.
So one can conceive induced drag from a 2D diagram either as a single vector resulting from raising AoA, or a decomposition to drag and lift.
Further study of Lift and Drag and their relationships at various AoA is highly recommended at airfoil tools on the web.


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Can turbulence happen at landing?
If we  exclude wake turbulence, can this turbulence happen at low altitude, for example at landing or take off?
How many meters plane drop in turbulence, if that happen at landing, plane will crash or this type of turbulence don´t happen at low altitude?
Here I find airliner drop during take off.

Yes, it is possible.  They are called microbursts, and are to be avoided for obvious reasons.
How many meters of sink, what will happen, etc. depends on the aircraft type and severity.

So if we avoid bad weather then no worry for turbulence at landing?

You can't say that. Microbursts can happen on good days as well. Here is a detailed write up of a recent glider crash due to a microburst  https://chessintheair.com/invisible-microburst-kills-expert-glider-pilot/

@EnginKurutepe, I wouldn't call winds gusting 40-50mph a "good day".  The glider crash you linked to was a tragic event, but let's not insinuate that it wasn't preventable or relates to hazards directly and commonly applicable to airline travelers.  Airline pilots brief and practice go-around procedures for low level wind shear.  Not saying it couldn't happen, but please don't stoke up more fear without adding some perspective or context.

@MichaelHall I find example where airliner is hit by microburst at take off, added to my post.

@user207141, interesting, thanks for sharing.

@MichaelHall if read the report the prevailing wind was 9 kts straight down the runway as reported by the eyewitness pilot who landed 5mins after the accident. The 40-50mph wind you mention was the microburst that caused the glider to fall out of the sky. Not trying to stroke any fear here. The question was if avoiding bad weather is enough to avoid turbulence and the answer is not categorically no. I wasn't aware the context was commercial aviation.


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Is "ALL CAPS"-handwriting mandatory in aviation?
All caps handwriting increase clarity and visual impact, engineers, architects, people from military mostly write in all caps.
I know lot of old school-engineers that handwrite in all caps, because in technical drawing it is mandatory.
Is writing in all caps regulated in aviation ?
(Do aerospace engineers mostly handwrite in all caps?)

The FAA requires that flight progress strips be marked using capital letters (with some exceptions to avoid misinterpretation, for example the letter "U" is written with a "tail"). 7110.65 FIG 2–3–1.

As Randomhead's comment highlights, "aviation" is an extremely broad category...  I can add that aircraft equipment certification plans and test reports are written in standard English without "yelling" in all caps.

Related: https://writing.stackexchange.com/q/64367/29719

Aircraft accident voice recording transcripts are written in all lower case.


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What are common drag and lift coefficients of aircrafts using frontal area as reference plane?
What are common drag and lift coefficients of aircrafts using frontal area as reference plane?
I want to know this numbers to see how actually streamlined are  planes compared with other objects, cars, etc..
I will write one example:
glider wing 20m2, frontal area 1,2-2.0m2
using wing area
Cd- 0,02-0,04
Cl - 1.0-1.5
using frontal area
scale by wing/frontal factor x12
Cd- 0,24-0,48
CL- 12-18
Cd of 0.48 is very bad result for object, so this numbers are missleading (even if we use same reference surface) when we compare object that produce lift(glider) with object that not produce lift (any shape). Because glider looks more streamlined than some boxy shape.

You just have to scale the coefficients by a factor $\frac{S_{frontal}}{S_{wing}}$

@sophit how do you mean? If wing area is 12x bigger then frontal area, that mean Cd and Cl will be 12x bigger when use frontal area as reference surface.

@sophit glider is 0.48, that is very bad for car...so this numbers are misleading when we compare object that produce lift with object that not produce lift...induced drag...

@sophit I think aerodynamics efficiency L/D is not missleading, even we use different reference surface..

Yes, L/D doesn't change and yes, if you compare the drag of a flying object with a rolling one the comparison is unfair (due to the induced drag)... But comparing the $C_{d_0}$ of an airplane with the $C_x$ of a car can be done

One can back calculate Cd of an aircraft from it's glide ratio, which corresponds to sine glide angle × weight = "thrust" (graviational component).  Thrust = Drag.  Most of the drag comes from the fuselage.

What are common drag and lift coefficients of aircraft using the frontal area as a reference? I want to know these numbers to see how actually streamlined are planes compared with other objects, cars, etc..

The comparison you want to do among automotive and aeronautical vehicles can be done but it is fair only considering a not-flying aircraft in order to avoid the induced drag which is something not existing in the automotive world.
Then we take only the parasite drag into consideration. This drag is anyway not proportional to the wing surface nor to the frontal area of the object rather to its wetted surface i.e. the surface directly in contact with the air: So if we really want to make a comparison among different bodies we should actually consider their wetted surface. This information on the wetted area is normally missing but it's not a problem since we can simply use a different kind of drag.
Like any aerodynamic force, drag is defined as:
$D=½ \rho V^2 S C_d$
Now, if the two vehicles are moving in the same air density $\rho$ and at the same speed $V$ then the only difference in drag $D$ comes from the product $SC_d$ which is termed "equivalent flat-plate drag" and is measured in m².
For example, according to Wikipedia a SUV can have a $C_x \times A$ as high as 1.6m²  while a Cessna C172 with a wing surface of 16.2m² and a plausible $C_{d_0}$ of 0.02 has a relevant value of 0.324m²: At the same density and speed, the Cessna needs a thrust 5 times smaller than a modern SUV.

race cars produce downforce, this car produce 17 000N downforce at 160km/h, has L/D 16:1   https://www.youtube.com/watch?v=YLIEeDL2J6M


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Does a wing fly in its own downdraft?
I find in many books that a wing flies in its own downdraft.
Does wing really fly in own downdraft and what does that mean?
Video Source 18:15 - 19:30

Perhaps OP means induced velocity ?

@sophit i edit post

First of all a clarification: what you call downdraft is more correctly termed downwash.

Wing fly in own downdraft?

No
The flow field around a wing might be complicated but in general it can be seen that, around a lifting wing, air:

goes upwards in front of the wing;
accelerates on the upper surface of the wing; and
decelerates on the belly;
goes downwards on the back of the wing.

If you could isolate those movements, you would see the air basically rotating around the airfoils of the wing.
So the wing isn't really flying in its downwash but it is actually producing a downwash behind itself. This is also stated in the video you linked.

So there's really no way for a wing to fly in its own downwash? Yes there is. This happens all the time for example in a helicopter rotor. Each blade generates a downwash it's going to collide with as soon as it has done a complete rotation.

author in video tell that downward motion happened even before wing(listen at 18:47)
... I see that downward motion is above,below and behind wing

I'm pretty sure that he just wanted to point out that the wing modifies the airflow already well in front of it and that the net effect is a downward movement (downwash). In front of the wing the airflow is pushed upward, as also visible many times in the wind tunnel tests in the video.

From max camber downstream, air has downward component, so we can say 3/4 of wing travel in own downwash

No because the airflow begins to go upwards well in front of the wing, so it's not really ¼ vs ¾. The net downwash is behind the wing, not on the wing.

Well, if you look at the Pistolesi theorem, the last quarter of wing chord is already in downdraft. But the first three quarters are in updraft.

A wing creates lift by pushing down on the surrounding air (some may debate whether pushing is the best word but let’s go with that for now).  This unavoidably makes the surrounding air move downwards, so much of the wing will be moving through air that is moving somewhat downward.  So it’s essentially true that a wing does fly in its own downwash.  That said, the wing is continuously in movement and so it’s always just on the edge of escaping from the downwash behind it, if it helps to visualise things that way.


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What type of probe can measure induced drag in the wake?
Induced drag can be measured from velocity preturbation in Treffz plane.

In Treffz plane what velocity direction is important for induced drag, vertically down, lateral(horizontal, "spanwise") or all possible angles?

What probe  I need to measure induced drag in the wake and how probe must be oriented?

How far from wing I must install probe, velocity die with distance from wing, so every location will get different magnitudes and directions?

If you measure flow field velocities, how do you split between e.g. friction drag and induced drag?


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Does twist increase spanwise flow?
Generally speaking, does twist increase spanwise flow, because high pressure at wing root stream toward lower pressure at the tip?
I will write example with sailing jib:
When sail upwind, jib windward telltales can stream up or at 45 degress up(often middle and top one), leewward stream all horizontal.
I cant figure out what change wind direction in upward direction at windward side of sail?
Here is possible solution:

Too much sail twist

Jib sweep angle

Jig  angled to leeward maybe cause this spanwise flow

Boat hull redirect wind in upward position

telltales: green windward, red leeward

EDIT: when I answered, the question consisted only in the very first paragraph so I didn't know yet it was actually a nautical question.

On fixed-wing aircraft twist is mainly used, together with chord variation, to obtain a lift distribution as elliptical as possible, in order to minimise induced drag.
Moving along the wingspan, everywhere lift changes a so called trailing-vortex is shed backward and contributes to the formation of the wake. This vortex is correlated to the bent of the flow on the upper and lower wing’s surface as visible in this picture:

Spanwise flow distortion. Source: https://cdn-images-1.medium.com/max/1600/0\*2gL-QOXqx51qNdAE.jpg
To simply summarise: lift changes spanwise $\Rightarrow$ trailing-vortex is shed in the wake $\Leftrightarrow$ flow on the wing bends.
So, does twist increase spanwise flow? It depends on if and how much this twist makes the lift change.
For example, on a rectangular wing with no twist, the lift changing is concentrated toward the tip and therefore the biggest spanwise flow has to be expected there. On a jetliner’s wing, the spanwise lift is more uniformly distributed and so is the spanwise flow.
The following picture shows a sample of different lift distributions:

Several spanwise lift distribution. Source: https://1.bp.blogspot.com/-e8479jPK0N0/U28fSOl8nrI/AAAAAAAASkM/35wCfWrvFq4/s1600/lift_distribution.jpg
The red line is for a rectangular wing; the target lift distribution is the elliptical one; and $\lambda=0$ denotes a delta wing.
From the picture it can also be understood how twist works: twisting the wing so that its tip has less incidence makes lift diminish there and a more ideal distribution is achieved. As pointed out by @RobertDiGiovanni this has also a relieving structural effect.

Bonus material
Sweep angle generates spanwise flow too: with a swept wing, the approaching speed can be decomposed in a component perpendicular to the leading edge plus a component parallel to it. This latter (called $V_n$ in the following picture) is completely spanwise flow:

Speed decomposition. Source: this answer
Twist is used also on rotary-wing aircrafts but whit the double effect of both optimising the bladewise lift distribution to minimise induced drag and to compensate for the rotating speed becoming smaller and smaller (until null) toward the root. So, also in this case everywhere the lift changes spanwise a trailing-vortex is shed in the wake and the flow on the blades's surface bends, just like in the first picture.
When the helicopter is flying forward and the blade, during its rotation, is passing over the nose or over the tailboom, the freestream flow is also investing the blade completely spanwise.

Downvoter: "Please consider adding a comment if you think this post can be improved."

This answer contains a lot of effort, but interestingly enough, twist lowers wing tip AoA, which may reduce the wing tip vortex (as well as giving the wing more desirable stall characteristics).  Elliptical lift distribution helps reduce wing tip (torque) stresses, but reduces overall lift (which is why some folks drop slats when heavy).  Spanwise flow is much more an issue with swept wings, hence the development of winglets.

@RobertDiGiovanni: thanks for your comment. Yes, the twist is used to reduce the wing tip vortex, I wrote it but maybe it is not clear, I'll adjust the text. Anyway I didn't consider any structural related issues here since it was not in the (original) question

This is a sailing question, but we must look predominately at the sail design, which comes to a very fine point, and also the speed of the wind and the boat, both much lower than typical aircraft.
So, there will be "spillage" of higher pressure to windward towards the finer end of the sail, while the leeward telltales are straight from the forward movement of the sailboat.
However, Reynolds number of this system is too low (Velocity) to generate an airplane-like circulation.
A square or rectangular jib would reduce spillage, but would capsize the boat easier (this is actually analogous to increased torque stress on the wing by not having an "elliptical lift distribution").
Wind tunnel studies would help here, but the spillage of pressure windward seems normal.

Yes, at least across the wing's lower surface.  For twists gentle enough to still have positive AoA at the tip, more twist means more AoA at the root, which means more pressure at the root's lower surface.  Air flows from high pressure regions to low pressure ones however it can.  The greater the pressure difference, the greater the flow.  QED.
To see for yourself, tape pieces of yarn to a cardboard "flat plate airfoil," grasp it by its root and tip, hold it in front of a fan, twist it, and watch how much the yarn deflects spanwise.


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Inudced drag distribution at eliptic and rectangular wing?
I am intersted in induced drag distribution at recnatugular and eliptic wing? It would be even better if graph show downwash distribution, lift distribution and stall behaviour for more wing shapes..
Why here lift and Cl distiribution are not the same?

Guys, please don't downvote without explaining why. It's so very, very rude. Downvoters: are you upset about the content? About the typos? About something else?

The local lift, usually lowercase $l$ is integrated to form the total lift $L$.
$L=\int_{-b/2}^{b/2} l\,dy$
Just as $L=C_L\, q\, S_{ref}$, $l=c_l\,q\,c$ where $c_l$ (note lower case) is used for 2D and it is multiplied by the local chord.
You might notice that
$S=\int_{-b/2}^{b/2} c\, dy$
So, in the figure above, the chart labeled $C_L$ should have been labeled $c_l$ and the chart labeled 'Lift' should have been labeled $l$.  This looks like a chart that may have been correct in its original form, but has been re-made so many times that over time someone didn't understand the importance of lower-case capitalization on those labels.
The reason the $c_l$ and $l$ charts do not look the same is because the chord of those wings varies.

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text:
Efficiency of the Kuznetsov NK-88 or NK-89 engine burning hydrogen
It's tough to find data about turbofan efficiency running on hydrogen.
The Tupolev Tu-155 has been used as a test bed for exotic fuels right before the fall of the soviet union, without much follow-up.
Is there any spec available on these engines? (like:)

Rated power
How much hydrogen used per second
Durability
Thermal efficiency

Matthieu Thomas of Airbus wrote an article stating an hydrogen turbofan could be more efficient than current kerosene ones because of:

Wide flammability range of hydrogen
High auto-ignition temperature

But as the piece is oriented pro-hydrogen, the author is probably not willing to admit any performance-wise downside of hydrogen when used in a turbofan. So a subsidiary question would be: Is there any foreseeable decrease of turbofan performance when switching to hydrogen?

There're two separate questions here (what were the NK-88/89's engine specs? / is there a possibility that switching a turbofan from kerosene to hydrogen could decrease its efficiency?); you might want to consider splitting one of those out into a separate question.

@Vikki-formerlySean you're right, I marked the efficiency decreased part as a secondary question and removed the highlight. I keep it there as the intended topic is hydrogen turbofan efficiency; the second part suggests a direction to go the extra mile

Based on my reading, the biggest disadvantage of hydrogen as an engine fuel is its low density.  Even when stored as a cryogenic liquid, it take up more than ten times the space as the same mass of kerosene, which means the same size tanks will hold only about a third as much energy (even though hydrogen has about three times the energy content per kilogram as kerosene).
Given these qualities, an engine suitably redesigned to burn hydrogen ought to be able to run more efficiently in terms of specific power output (energy produced per mass of fuel) or specific fuel consumption (same thing only upside down -- fuel consumed per energy produced), but even ignoring the weight of insulation and the fuel cost of boil-off in flight (or before takeoff), an aircraft the same size would have roundly a third of the range.  This is usually taken to mean that hydrogen fueled aircraft would have the same issues as hydrogen fueled rockets: they have to be very large to carry an economically useful amount of fuel.
There are other methods of storing hydrogen (developed for automobile prototypes); as I recall, the highest density of storage (kilograms of usable hydrogen per volume) has been with metal hydrides; these require neither very high or very low temperatures, and operate at reasonable pressures (rather than hundreds of atmospheres as common pressure bottles do) -- but they barely exceed the density of liquid hydrogen, and there's a penalty in the weight of an "empty" tank, which is still filled with a solid material when no more hydrogen can be drawn from it.
Beyond qustions of range, hydrogen is difficult to work with in terms of materials (it causes most metals to become brittle just from being in contact with hydrogen), it leaks through almost everything, and is so flammable that these unavoidable leaks are very prone to become invisible conflagrations (because a hydrogen fire contains nothing that glows from heat, the flame is practically invisible -- the bright flames of the Hindenburg were due to the envelope and its coating, not the hydrogen).  This is obviously irrelevant to engine efficiency, but very relevant to whether such an engine can last long enough in service to be economical to operate.

I agree on everything you say. tbh, I'm looking at efficiency figures to run calculations and show with figures how unpractical hydrogen is anyway, for those who don't comprehend what you mentioned. For instance, replacing kerosene with hydrogen in an A350-900 cargo would eat up all the storage room the plane has to offer and still lack 260 cubic meter to store all the hydrogen needed to pack as much energy as kerosene does. In the process, you'd save 70 tons of weight. With engine efficiency figures, I could more precisely calculate how much hydrogen you'd need to bring for the same flight.


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What amount of electricity could we get from the Seebeck effect on a kerosene turbofan?
Edit: I rewrote my post. To downvoters, can you comment on your reasons? As a kind reminder:

Downvote questions that don't show any research effort or don't contain enough information to be clear and answerable. These questions may also need to be closed.

The Seebeck effect is used to power space probes that travel too far for solar panels to be sufficient. In that scenario, the heat source is provided by radioactive decay, which lasts for decades. More info on Radioisotope thermoelectric generator and the example of Voyager 2.
The voyager probes had 3 RTG - 99.6kg total without the plutonium - for a rated power of 470W. Keep in mind that this device can have a structural value and therefore an inferior net weight. Also, those are values from the 1970s.
The Seebeck effect is the first option that came into my mind to produce electricity from the waste heat, of turbofans or other areas with unwanted heat (pumps, radar?). The point is to recover wasted energy (other ideas welcome).
Electricity consumption figures of the airplane systems (link provided by Bianfable) are currently around 90kW for essential load and utility systems (basis for comparison, keep in mind these values can be reduced).

Related: How would exhaust heat recovery into the combustion chamber affect the efficiency of a jet engine?

I thought aeroplanes didn't have separate onboard generators with fuel? I thought the engines were connected to generators?

@user253751 They are, usually via the accessory gearbox connected to the high pressure turbine. But that energy is not for free: since the generator is driven by the turbine, you need more fuel fuel to achieve the same engine rotation.

@Bianfable doesn't slower exhaust also mean you need more fuel too? according to the related question

@user253751 Yes, that's probably one of the reasons why this heat recovery is not done. But in a high bypass turbofan most of the thrust is from the bypass air. If you only reduce exhaust speed in the core, this might still work. I'm not really qualified to say if it's actually a good idea, though. Maybe someone who understands more about it than me will answer...

@Bianfable you'd probably cool the cold side of the generator with bypass air.  This might affect the flow and therefore the efficiency

Powering the electric needs of a jet aircraft with thermocouples? The supply & the demand are mismatched here by a few orders of magnitude, it seems. It takes some pretty serious amps to run the 2 electric hydraulic pumps in a 737, plus radar & various computers."Silly to have an on-board generator"? Ah, no. -1

@RalphJ Buffer batteries can provide for peak demand. How much Wh do hydraulic pumps use on average during the highest demanding part of a flight? Same Q for radar (did not find the power rating of the 737 radar)? Computers can run on a few tens of watts.

@FlorentHenry This answer has a nice diagram illustrating required electrical power for a single aisle aircraft. It averages ~100kVA!

Both electric hydraulic pumps run full time throughout the flight as long as any generator is operating. They do not, however, run in the (emergency) case of all generators failed - they'd drain the battery far too quickly.  The power generated by a thermocouple is wildly out of proportion to the power used  by the aircraft. Until the understanding of the math involved enters the picture, this is all in the realm of science-fantasy.

@Bianfable thanks, I included it my post

@Florent Henry, point taken.  I removed both my downvote and the earlier comment.  I’m not qualified to access whether it deserves an upvote, but we need relentless challenge to the status quo to drive continuous improvement, and I appreciate your willingness to respond to feedback.

@RalphJ Why do they run continuously if they don't need to during an emergency? (Please keep in mind that the question is to get a value of how much energy can be recovered from wasted energy in the prospect of replacing generators.)

@MichaelHall Thank you for reflecting on it. I've cleaned up my replies to you.

The amount of current flow produced by a seebeck effect device is tiny relative to their size. You would need a plane load of them just to power the lighting system in a passenger jet.

Any figure of the lighting system power requirements?

an estimate would be around a thousand watts.

You can't escape the tyranny of the law of conservation of energy and the second law of thermodynamics (with it's corollary limiting thermodynamic cycle efficiency).
The energy taken from cooling in a reciprocating engine is truly wasted, so the thermal gradient there can be further used without affecting efficiency of the engine itself. But turbines have very little external cooling; only a bit for the oil. Most of the heat is kept in the working fluid by cleverly directing the air around and into the combustors.
And if I understand the thermodynamic right, the heat in the exhaust gas is the part that must be ‘transferred to the low temperature reservoir’ for the cycle to work, and therefore tapping into it will reduce the efficiency of the engine.
Now WP says about the efficiency of thermoelectric generators:

Currently, ATEGs are about 5% efficient. However, advancements in thin-film and quantum well technologies could increase efficiency up to 15% in the future.

That's very, very poor compared to an alternator.

They're of relative poor efficiency but they'd be running on waste heat anyway.

@FlorentHenry Most waste heat is contained in the exhaust flow and not really wasted. If you cool down that flow, you would extract most of the momentum it has gained in order to push the airplane forward. So, no, that's a reservoir you better leave untapped.

The context is that commercial airplanes make up most of the traffic.

Let's take the A320 as an example of commercial airplane.
http://a320dp.com/A320_DP/electrical/sys-3.7.1.html

The main AC electrical power sources are:

Left and right engine integrated drive generators (IDGs) (90-KVA each)

KVA is kiloVolt-Ampere, roughly equivalent to KW.
So we need about 180KW. From you question:

The voyager probes had 3 RTG - 99.6kg total without the plutonium - for a rated power of 470W.

So $180000/470 \approx 383$, thus $383 \cdot 99.6 \approx 38145 kg$.
That's extra 38 TONS.
For an aircraft whose maximum payload is just shy of 20 tons.

Addendum: thanks to Brianfable for finding this other question with extra data on power generators.

(I edited the question, figures are from the 70s). 180kW is for redundancy, 100kW would be a more accurate starting point. Supposing we can crank it up to 600W, that's 16.6 tons. Using 1/3 of it as structural elements, we're looking at 11.1 tons, from which we should remove generators and some of the fuel weight (I have no plausible value to offer here). And we've lost redundancy. But it changes many more things not taken into account here. Providing only part of the electricity with a smaller system could prove more efficient overall anyway. Q remains, how much can be practically recovered?

@FlorentHenry nothing. nothing is practically recoverable. seebeck effect requires too much weight per watt to be practical for aviation. solve that first and then the answer might change.


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Need help identifying civilian small aircraft from old turkish movie
I need help identifying a plane from an older 1977 turkish comedy movie for a personal project, these are the best (and only) shots it can be seen.
Flyby
Front Angle
Close up cockpit outside

It is a Cessna 188 crop duster. The giveaway is the shape of the cooling air inlets on either side of the prop, which are "Cessna-shaped", as opposed to those on the similar Piper Pawnee which are rounded.

Yep that's it nice catch.

This answer is a bit short but nevertheless correct.

@PeterKämpf, detail added! -NN


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Can anyone tell me more info about this radial engine?
non-aviation expert here doing a project for a friend.
I am trying to learn a little more info about this 3D model radial engine.  I downloaded it off a stock 3D model website, that offered almost no information, specifications, etc included.  After doing some research, I think this is a two stroke, supercharged, 8 cylinder radial engine, with each cylinder firing sequentially: from left to right(not the usual alternating firing order of radial engines).  The kind of engine that would be used for a small aircraft, and possibly from an aircraft that came from the first half of the 20th century.
In the image below, I just need a little help in identifying some of the specs.  If anyone could help me learn a little more information about this engine, such as, type(example: propulsion reciprocating & rotary), dimensions, weight, power rating, operation speed, displacement, materials, etc….i would be forever in your debt!!!!  I apologize if the pictures are not the best, and want to thank you for any help you can provide!

Is there any reason you believe this random 3D model is based on an existing engine?

It looks like a version of this 3D model which has no reference to a real life one.

@Sanchises
You have a great point! I honestly didn't think of the possibility that the 3d model I am using was a real concept/patent that was actually manufactured, distributed, and used in a number of different applications at some point.

@Sanchises
however with the exception of applications for model airplanes – and based on my own bias in thinking this particular 3d model might have more in common with an older two stroke radial engine - would you know if this model would be comparable to something like a KHD Dz 700: a two stroke, eight cylinder diesel engine manufactured in the first half of the 1900’s?
Any help would be appreciated!

@mins  thats it! and yeah, no real life reference.  it be like that sometimes i guess.

@jerkules Interesting find but that engine is in no way comparable to a normal radial. It's fitted with two opposing pistons to overcome the valve timing issues mentioned in the other question.

It is unlikely to represent a real radial engine because it has an even number of cylinders. For reasons explained elsewhere in this SE, radials must always possess an odd number of cylinders.

There are 2-stroke engines with sequential firing that can have an even number of cylinders, but these are generally seen only in model aircraft sizes, often using cylinders, heads, and pistons from single-cylinder engines -- and I've never been sure how they manage to produce crankcase pressure to force induction unless all the pistons are synchronized to TDC.

@ZeissIkon, also note this engine has pushrods and rocker arms, so it is not one of the 2-stroke "smokers".

Very unlikely.  There are uniflow 2-stroke diesels with intake valves, but they use superchargers of one sort or another to provide induction pressure.

@nielsnielsen
From my research, a radial two stroke engine will have the cylinders fire on every revolution, so having an even number of cylinders is not an issue.
Also after researching a little, it seems like a modern example of what im talking about would be the two stroke Zoche diesel radial.

@nielsnielsen
With that being said, its been brought to my attention that this particular 3d model radial engine I posted about may not be based on a real concept/patent, however with the exception of applications for model airplanes – and based on my own bias in thinking this particular 3d model might have more in common with an older two stroke radial engine - would you know if this model would be comparable to something like a KHD Dz 700: a two stroke, eight cylinder diesel engine manufactured in the first half of the 1900’s?
Any help would be appreciated!

@jerkules, I am sorry, I do not.

@ZeissIkon
with the exception of applications for model airplanes – and based on my own bias in thinking this particular 3d model might have more in common with an older two stroke radial engine - would you know if this model would be comparable to something like a KHD Dz 700: a two stroke, eight cylinder diesel engine manufactured in the first half of the 1900’s?  Things like dimensions, weight, power rating, operation speed, displacement, materials?  Any help would be appreciated!

@jerkules Can't tell you anything about that KHD -- but there have been very few diesel engines made for aircraft, in part because of temperature limitations (if intake air is too cold, compression can't heat it to ignition temperature and the engine stops working) and in part because aviation fuel distribution is all aimed at high octane gasoline and jet kerosene (the latter of which is used as fuel in the two current aircraft diesels I know of, though they can also run on highway diesel).


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How is it an advantage for the L1011 to not right itself?

A friend who flew the L1011s loved them because of their stability. His joke was that you could put the plane into a banked turn , go to the bathroom, come back, and you would still be in the same banked turn. He felt that other planes like the 767 require far more concentration.

Quote from Source
Can a pilot explain to me how an aircraft not returning to straight and level flight on it's own decreases workload? Would you not have to correct every small bank error just to not go off course?
Edit Jan Hudec is correct when he says that it may involve s comparison of spiral dive tendency. But

still be in the same *banked" turn. H

Makes me wonder why it doesn't right itself

Why do you think it's being compared with returning to wings-level? It's being compared to continuing to bank into the graveyard spiral.

@JanHudec well, I see no evidence that it doesn't work both ways. But yes, comparing the he spiral behaviour makes sense

It's because, while neutral roll stability (tendency to stay at a bank angle until disturbed) is an undesirable characteristic for an aircraft that is flying straight and level because more attention is required to keep it there, it's a desirable characteristic while maneuvering.
Normally, you have to choose one or the other, so you choose positive roll stability because that's the priority for an airplane designed to cruise.
Airplanes that spend a lot of time, or are optimized for maneuvering, like aerobatic aircraft and gliders, tend to make neutral roll stability a priority.  This is a really desirable feature on a glider that spends perhaps a majority of its time in thermals in 30 degree banked turns.  When you spend extended periods turning, the tendency to hold a bank angle is a great feature.
As well, when flying on instruments, a tendency to hold a bank angle naturally is a useful feature.  You are generally either in level flight where you want positive roll stability, or are in a standard rate turn, where a tendency to retain the turn rate without input is nice to have.
So, an airplane that had positive roll stability while level and neutral roll stability while maneuvering (on purpose) can be the best of both worlds.


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What does the Permanent Magnet Alternator do exactly and how works?
From T6b Naval manual:
A dedicated permanent magnet alternator (PMA), mounted on the reduction gearbox, powers the PMU. The PMA supplies 32 VAC, which the PMU converts to DC. The PMU automatically switches to the 28 VDC battery bus when propeller RPM drops below 40-50% NP, or when the PMA fails.

What does the PMA really do? From looking online I only found that in other engines the PMA is good to convert mechanical energy into electrical not sure if this is the case for the PT6 engine.
I think it has to be with the startup process, not sure. During the startup the aircraft its connected to an Electrical Power source if battery is low but the engine does not require any gear accessory connection, only electrical power. I have seen how big jet engines need an accessory gear connection go turn the compressor, how does this PT6 engine actually starts? Im guessing electrical power somehow moves the compressor shaft?
The PT6-68 engine has an Starter/Generator that acts as an starter and then a generator, which is aft the accessory gear box. Is this related to the PMA? Does the starter somehow moves the compressor shaft?
I couldn’t find much in The P&W manual, its in the link below. Thank u in advance
https://mikeklochcfi.files.wordpress.com/2018/08/training-pt6a-60-series.pdf

A permanent magnet alternator is just an alternator. It produces electricity and charges the battery while the engine is running. The type of alternator is a largely irrelevant detail here, but in case you're interested:

A permanent magnet alternator is maintenance-free, but output voltage is proportional to rpm, which is undesirable for most applications.

A conventional alternator has a spinning electromagnet, powered from the battery via brushes. The brushes wear out and must be replaced occasionally. This setup allows magnetic field strength to be adjusted by a regulator, so that the alternator produces a constant voltage regardless of rpm.

Thank you for the input. But based on the statement from the book, it looks like the PMU can get its power from the battery bus, so why putting an alternator at all, If you could power the PMU with the battery bus that is being powered by the Generator whenever the aircraft is working properly?

@YamchaAviator - Is there a separate generator/alternator somewhere else or is this the only one? If it is not the only one, then maybe for backup electricity when the battery bus fails while the engine is running?

@YamchaAviator - By the way, what is a PMU? You use that abbreviation a lot without ever explaining it.

@YamchaAviator Redundancy. You need to use ship's power/battery to power the engine electronics during start (and maintenance), when the PMA isn't spinning. Once the PMA reaches a minimum speed, the EEC/FADEC transfers over to it. If either the ship's power bus or PMA fails, engine operation continues with the operational source. I believe some engines also read PMA frequency as a tachometer, instead of/redundant with a separate sensor.


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Why is it that low fuel gauge is one indication of a compressor stall?
I am reading from my gas turbine engine class notes that apparently, one indication of compressor stall (besides the banging sounds, torque gauge low, N1 low, ITT gauge up, etc) is the fuel levels starting to drop.

since the compressor stall results in a reduction of airflow to the
turbines, more fuel will be required to maintain the current thrust,
increasing the fuel consumption increases burner and turbine temp

Ok, so (the last paragraph was from the class notes) yes I would agree that due to the lack of enough airflow turbines and burner sections might increase in temperature.
However, why is there more fuel being released to the combustion chamber whenever there is less air? shouldn't the fuel control unit or another system reduce the fuel due to the reduction/lack of air to keep the ratio of air:fuel ratio for a proper mixture?

in the title and first paragraph of the question, you are talking about a decrease in fuel flow, but then in the second and third, you are talking about an increase in fuel flow.  So I'm not really sure what you are trying to ask about.

that's exactly what I mean. title and first paragraph apparently are facts that's what happens (fuel composition goes up, fuel reading or gauge goes down). however, I don't understand why that is the case when I would suppose that the engine lowers the release/comsuption of fuel to keep a good air:fuel ratio.

Maybe you think that fuel is measured to achieve the correct stoichiometric ratio (as usually done in piston engines), but this is not true for gas turbines. They run much leaner so that part of the oxygen is only working mass (as is all of the nitrogen) and much more fuel could be added before the mixture becomes too rich. This is avoided, however, to keep the turbine entry temperature below its limits.

Re "increasing the fuel consumption" and "one indication of compressor stall ... is the fuel levels starting to drop."-- So you are suggesting that the fuel consumption increases so much that the pilot is able to notice that the fuel tank gauges are moving toward empty at a higher-than-usual rate?  That seems rather implausible.  There are really two questions here-- I would suggest editing to focus entirely on " why is there more fuel being released to the combustion chamber whenever there is less air?" and forget about the fuel tank gauges.

(Ctd) Once you have answer to the first, if you then want to ask whether rapidly-lowering fuel tank gauges really are a practical, useful indication of a compressor stall, you could ask that as a separate question.

I might depend on what your fuel control unit is designed to do, and maintaining the optimal fuel/air ration may not be it.  For example, some Full Authority Digital Engine Controls will look at power lever position and meter fuel to achieve the pilot desired power output, therefore adding fuel when airflow decreases.
This would be a good question to ask your instructor if your course is engine specific.  I will also add that a lower than normal fuel gauge quantity indication will be one of the very last indications of a compressor stall that you will notice!

awesome Michael. That last sentence "adding fuel when airflow decreases" ... meaning adding more fuel to what was already being added? or is it adding the same amount of fuel and now with a lack of enough air it looks like is consuming a lot?   or how does it make sense to throw more fuel to a tiny bit of air and expect moe thrust? I guess that is where my confusion is.

I just learned today that the FADEC I will be flying will add fuel when bleed air is turned on in order to maintain pilot demanded power output.  So, with a decrease in airflow that would naturally cause the fuel air ratio to change.  But it kinda has to be that way, because if it reduced the fuel flow further to maintain the ratio the power output loss would be even more dramatic than if it did nothing at all, right?

ok yes, I am starting to see the light. I think it works similar to a car, when your air filter is dirty or basically not enough air coming into the engine your car does consume more fuel, the question is why? I guess I have to look at the chemical reaction of it?

Well, you touched on it already, if the mixture is rich more fuel will be used for a given power output, right?

right, but shouldnt just keep consuming what was already consuming? why more though?

If something is constant, (in this case power) and you change one variable, (in this case reduced air) then something else must change in order to maintain the constant.  (in this case fuel)  Otherwise if you reduced airflow without changing fuel flow the power output would decrease.  In a car, as your filter gets dirty, you don't just drive slower, you push the gas pedal down more to drive the same speed.


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Does wash-out reduce drag?
I am in the process of designing a small wing (Re ~ 50-100k) and was planning to "un-twist" the tip of the foil by -5°. This corresponds to the 5° aoa where the center part of the wing has the highest Cl/Cd.
I am aware that wash-out is mostly about keeping the wing tips out of stall longer than the rest of the wing, but intuitively having the wing tips at 0° aoa during normal operations should reduce the amount of "redirected air" at the wing tips. This in turn should reduce the strength of the wing tip vortices, which in turn should reduce drag?
edit for clarification: I intent to have the tip of the wing be at 0° aoa with an airfoil that does not generate any lift during normal working conditions.

I've seen wingtips create downward lift due to excessive washout-- that did not reduce drag!

@quietflyer I intent to keep the tip of the wing "perfectly" aligned at 0° aoa during normal operation. If that works out there should be no downward lift generated there.

@fho - If they produce no lift then they are dead weight. Why extend the wing if it’s not going to do anything?

@Jim I was under the impression that reducing the lift along the wing span would have a similar effect as winglets. Both should reduce the impact of wing tip vortices.

The wake vortex is the shear between the air redirected downward by the wing—which balances the momentum given to the aircraft to counter gravity and keep it flying—and the unaffected air outside the span. By not producing lift near the tips you are effectively sacrificing some of your span, which means you need stronger downwash in the middle and that increases the drag, not decreases.
The most efficient configuration (for given span) is when the lift distribution is elliptical. The lift coefficient reduces towards the tips, but only gradually and never goes down to zero. There are many explanations for that around the site already. Try starting e.g. with For the elliptical wing, which property is actually elliptically distributed? (Peter Kämpf's answers are fairly well linked together so you'll find further references there).

Note that if you're concerned about stall, you should avoid a pure elliptical wing: when the wing stalls, the whole wing stalls at once.


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Why large freighter planes still do not use intermodal (ship) containers given that size and shape are not problems?
Recently I found this article this article about the concept of directly sit on a pile of intermodal containers and lock them in place. Whether it is plausible to open such a huge opening ventral of such monstority, I found the idea of using intermodals as air freights attractive, as it eliminates the hurdle of taking the items two and fro between two incompatable systems.

Also similar but more radically engineered are the spanloader concepts, where the intermodal containers are just tucked into the wide, rectangular fuselages, or even into chords of giant flying wings themselves. Please refer to this thread (in which many of links require Wayback Machine to access and takes time to load alrge pdfs) to see their applications including even fitting ICBMs inside.

There are even far smaller concepts of ground effect freighter which is just one container-sized.
Then I am curious: why the current jumbo freighters, instead of unsightily truncated ULD, do not directly load intermodal containers onto their cargo decks?
P.S. Size and shape of fuselage should not be the problem, as such odd shapes were meant for thinner cabins like this answer, while not a problem for new-gen wide bodies like this, not to mention jets modified to carry oversized cargo.

Look up relative fuel efficiency of freight aircraft vs. trains and tractor-trailer trucks.  Then compare that hoverfreighter's consumption.

You're talking about steel freight containers that easily weigh 2 1/2 tons empty?

There are probably multiple reasons, but one big one is weight. A standard shipping container is made of solid metal and weighs several ton(ne)s empty. An air cargo pallet on the other hand weighs only a few hundred pounds.
Given how critical weight is to aviation costs, transporting around tons of 'dead weight' in the form of steel shipping containers doesn't seem very attractive.

I am comparing intermodal with Unit Loading Device (ULC), which are basically aluminum cargo containers. A bit of Googling and I found there are several retailing companies selling aluminum containers in addition to Corten.

@CrystallizedRefresher I'm not sure I understand what you're comparing. Your third link shows that even an aluminum container weighs around 2000kg empty.

The ULC in average have effiency twice by weight and thrice by space compared to aluminum shipping container! I don't get the reason behind such advantage. Is square-cube rule working at this scale?

@crystallizedrefresher it is probably down to the fact that shipping containers are designed for hard usage - they are expected to be banged around, sprayed with seawater, cope with having other containers stacked on top for weeks, etc. A container for aircraft holds doesn't have to put up with this so can be a lot less robust - and thus lighter.

The size and the shape of the fuselage are a problem.
Almost all large freighters currently in operation are cargo versions of passenger aircraft. This means they share the circular or elliptical cross-section of the original passenger aircraft.
This saves a lot of money when retired airliners are reused for freight.
Even when they don't come from an airliner, e.g. the An-225 Mriya, an elliptical fuselage is very practical both structurally and aerodynamically. It can also be pressurized, which is relevant for some types of cargo.
Container ships don't have the aerodynamics issue, nor the pressurization one. The submerged part is streamlined, the top doesn't have to be. Also, they're far larger, so there's no reason to shape the containers as anything other than a box.
It's completely technically feasible to produce an air freighter shaped to efficiently fit intermodal containers. The containers themselves can be readily made lighter by using aluminum walls and ultra-high-strength steel beams.
A study into CFRP containers has shown that a 40' can weigh less than 1 ton. There are some issues with ultra-lightweight containers, e.g., a steel container doubles as a security safe. They can be overcome, and the time savings could be worth it.
But an aircraft designed for this role would be unpressurized, useful for intermodal freight and nothing else. There just isn't enough demand to support the development and certification of such a dedicated cargo-only aircraft.
So, currently, the chosen solution is to use ULD for air freight, which maximize the use of aircraft internal volume, and then spend extra time transloading cargo from and into trucks.

Modern container ships aren’t particularly streamlined to be honest - fairly cuboid shapes to help pack as many containers in as possible


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What is the origin of the following aviation slang terms?
I'm researching some aviation slang terms. I have difficulty in finding the origin of the following words and phrases. I would be grateful if you can help me. The words and the phrases and their meanings are:

Kick the tires and light the fires =  take off
Cookers = afterburners
Bird = a plane
Copy Shot = acknowledged
Dirty up = Lower the undercarriage
Hot rock = hotshot= a highly skilled pilot
Chop and drop = bring the power to idle and descent rapidly
Running on fumes = near the point of total fuel exhaustion
Conga line = a long queue of airplanes
Dry/wet feet = it alerts ATCs when a military aircraft is flying over water/ over land.
Bag= flight suit

That's going to be a tough one.  I would say most of these terms originated in the military in the 40s and 50s.  Suggest contacting USAF public relations to ask if there is any historical guidance from them, or if they can steer you somewhere useful.

The History or English language sites might be a better bet.  FWIW, I wouldn't say that any of those are in wide use in aviation.  (Maybe in some branch of the military?)  The only one I've heard - "running on fumes" - has been in an automotive context, not aviation.  And of course "conga line" as related to dancing, but there it's hardly slang.

You want exact origins, or just what they relate to? Finding the origin of most of those will be impossible, as they date back so far, that the "inventor" of the phrase can't be verified.

I voted to keep this open, and I think this is an excellent opportynity for community wiki, so...

Thank you for the replies. I know what these phrases mean. I am interested in the origin- when they were first used, why they were used, etc.

@Val: If you do know what they mean, perhaps you could edit that information into your question?  I doubt that I'm the only one who has no idea what most of them are supposed to mean.

Ok @Val, I suggest we keep the community wiki open, because i think this will be interesting even though exact origins are not traceable for most.

@Jpe61, I agree. I want to thank everyone who has tried to help me.

I have made this answer a community wiki, so we can avoid pointshopping and flood of conflicting answer. Only edit this if you are sure of what you think you know.
Kick the tires and light the fires
This phrase refers to the pre flight inspection, kicking the tires, a pilot performs on his/her aircraft before hopping in and starting the engines, the fires part comes from old planes having very short exhaust manifolds, so flames could often be seen at the tips of the headers on startup with fuel priming and rich fuel setting.

Cookers

Bird
Planes fly, birds fly; planes have wings, birds have wings, need we dig in more...

Copy Shot
Copy Shot = Unknown, although maybe referring to a carrier catapult launch or an aircraft missile launch.
This may come from a multi-aircraft flight during combat, or while flying top cover where an aggressor situation happens. "Lined up for a shot" (missile lock), then "missile's away" (launched missile) and the flight wingman might respond "copy shot" acknowledging the other plane has launched a missile.
May also relate to an aircraft carrier, where being launched from a catapult, is being "shot."

Dirty up
Dirty or Dirty Up = Extension of gear, hook, flaps, etc. for slow speed flight or landing
Flying clean means that as much of the components are retracted or flush as possible, allowing for clean air flow. Dirtying up refers to the acts of preparing for flying slow, or landing. Deploying the slats, flaps, landing gear, tailhook, etc.
Also used during the Vietnam conflict by pilots of small reconnaissance plans while flying slow and (relatively) low while searching for downed pilots, to let the controllers know they were beginning the search and more vulnerable.

Hot rock

Chop and drop
Simple combination of chopping power and dropping altitude. "To chop" is slang for "to cut", with "cutting" refering to "cutting power", therefore reducing power to zero. Dropping altitude is self explanatory, meaning to quickly reduce altitude. I imagine this phrase was formed like this because it rhymes.

Running on fumes
I would not call this one slang as it is basically a literal translation. In general it means to keep going when you have no energy left. It broadly comes from engines that ran on vaporous fuels; piston engines and the such. Most fuel tanks that hold liquid fuel have some sort of a float which reads the level, the phrase comes from the idea of running the tank "dry" or empty to the point where the float bottoms out (or reads empty) but the engine still runs. Its based on the loose idea that the free area in a fuel tank is filled with vaporous fuel and you could run the engine on the vapors at the end of your fuel tank since the final step of most fuel systems is to vaporize or atomize fuel anyway.
Ironically many modern planes may very well have a fuel inerting system that would quite literally prevent you from running on fumes. A lot of fuel pumps are also cooled by the fuel that flows through them so starving a pump of liquid fuel can cause it to burn out.

Conga line
Conga line is a dance people engage in when they have consumed enough alcohol to impair their judgement. The "dancers" form a line, holding the next one on the waist, and they proceed more or less rhytmically more or less forward in what can best be discribed as a squigly manner.
With aircraft, a conga line is a formation of aircraft, proceeding at a common general heading one after another, but not in a military like precicion when it comes to distancing and  track.

Dry/wet feet
Feet Wet : Flying over water
Feet Dry : Flying over land
These two phrases were used extensively during both the Korean and Vietnam Wars by pilots when they flew into the country (feet dry) and then upon returning to their carrier or air base, calling "feet wet" as they passed the coast and flew over the ocean.

Bag= flight suit
Use of Bag to describe the flight suit comes from its saggy fit. The fabric, at least in older ones, was certainly not stretchy, so the form of the suit had to be such, that it felt comfortable when sitting and operating knobs, buttons and such in cockpit. This, unfortunately, made the suits look saggy and baggy when the pilot was posing heroicly by the aircraft...

I thought conga line might have come from the Berlin Airlift, but could not find any references using that phrase.

Might be, I've heard it used in many occasions to describe a meandering line (that's the word I was looking for when I made my post) of anything: ppl, cars, birds, racing sailboats, ants... you name it, they conga

Post references for your answers above.  Without references, your answers are little more than the educated guesses.  Helpful, but not really


text:
Is it safe to carry bear spray on a GA plane?
I'm aware that bear sprays are normally banned from commercial flights, including in checked bags. As I'm not sure of the reason behind that, I'm in turn unsure of whether it's wise to have some with me in my Cessna as I take it to go backpack.
Are there risks to having bear spray with me, flying up to 12,500 ft?
Additionally, why is bear spray banned from commercial flights (I mean in checked bags, it's pretty obvious why they don't want that in the cabin)? Is it the difference of pressure? Surely these things are made to withstand pressures up to 13 or 14k feet, as they could very well be taken that high when simply hiking?

When you're hiking, you can toss the can of bear spray safely away from you if it starts leaking.  Try doing that in an enclosed airplane cabin.

Interesting your perspective.  The reason it's outlawed in commercial flights is how useful it would be to terrorists trying to take over the airplane.

@Harper-ReinstateMonica Also, the chance of encountering any bears at 10,000+ ft. is pretty much nil, so there aren't too many legitimate reasons to have it accessible during the flight...

@Harper-ReinstateMonica I meant why is it forbidden to have in checked bags, where it's not accessible during the flight.

Imagine being an airline manager, responsible for passenger safety in a multi-million dollar (a 777 is 306 million) aircraft -- and their luggage.  A passenger puts bear spray in their luggage, maybe leaky or a broken anti-discharge safety (dropped on rocks, sabotaged as a prank, whatever) and it discharges... into the middle of an aircraft hold full of luggage.  It all has to be cleaned, passengers placated and compensated, and a 777 costs around $7,380 per hour to operate.  You want to delay flights, handle that cleaning job, all the fallout?  Would YOU say "sure, no problem, pack it"?

The main danger in a light aircraft is that the spray accidentally discharges in the cabin. That could temporarily blind and incapacitate the pilot, which would obviously be extremely dangerous.
According to an article from the Alaska Department of Fish and Game, GA operators typically take special precautions when transporting bear spray:

You can take bear spray, but you must tell the carrier and the pilot
must know. It requires special handling. Carl Ramseth is with Alaska
Seaplanes, a Southeast Alaska regional service, which has planes on
both floats and wheels. Bear spray is defined as a hazardous material,
he said.
“You can fly with it, but it needs to be declared,” he said. “We ask
passengers as they check in – people on scheduled flights as well as
people going to remote locations. People don’t realize it could be a
problem, so we always ask. There is paperwork filled out. We transport
it in a sealed can, stored in a part of the aircraft that is not in
the main cabin, so there is no risk of it going off in the cabin. Or
it goes in the float compartment of a float plane.”
Some carriers use a length of heavy duty black PVC sewer pipe, four
inches in diameter and threaded with caps at both ends. Several cans
can be stacked and sealed in the tube for transport.

Another article on the same site says that some operators don't want to carry it at all:

“Bear spray is a big one, it’s dangerous,” said Stewart of Skagway
Air. “We can do it, but we really prefer not to. Typically speaking,
most carriers won’t allow it. You don’t want that going off in an
aircraft.”
“We really try to have them leave it behind,” said Vanessa Thompson,
with 40-mile Air.

Even if you don't have a floatplane, as long as you have some duct tape it's still possible to carry the spray outside the cabin:

ilya_ktsn, CC BY 2.0, via Wikimedia Commons
It might look odd, but it's allowed by the FAA per JO 8400.34 - Fixed-Wing External Loads in Alaska:

Items in this group are small, lightweight, and temporarily attached
with bungee cords, straps, ropes, etc. Below is a list of items that
are examples that may  be carried under a Standard Airworthiness
Certificate. For the purpose of this order, these
temporarily-attached items are not considered design changes or
alterations.
[...]
Cans of animal repellent, such as bear spray or pepper spray.

Very interesting, thanks a lot!

This is a way better answer to the main question than mine, since it brings out the real world comments about the bear spray. All in all, if pros don't want to fly with that stuff in the cabin, well...

Remember that even pepper spray for use on humans can easily incapacitate a human (that being, after all, its primary intended purpose), and bear spray is much more potent than human spray.

I just did this a few weeks ago with another Southeast Alaska regional service. They put the bear spray in a container in the float (the kerosene went in the float too). This is how I've always seen it done here.

ah, duct tape. The universal cure where WD40 is inappropriate :)

Now let's go crop-dust some bears!

@jwenting Seems like it should be speed tape in aviation, though. - haha

Second things first (just because I can):
Bear sprays such as this Wikipedia example are banned as carry on luggage on airliners for two reasons:

They are aerosols, which generally are banned
They contain capsaisine, meaning they essentially are a pepper spray, and could be used as a weapon.

TSA has ruled that:

One 4 fl. oz. (118 ml) container of mace or pepper spray is permitted in checked baggage provided it is equipped with a safety mechanism to prevent accidental discharge. Self-defense sprays containing more than 2 percent by mass of tear gas (CS or CN) are prohibited in checked baggage.

I am unaware of any rule or regulation banning bear sprays on general aviation aircraft, but unless the container was equipped with a safety device described in TSA rule, I would not take one on a plane.
Not that I would be worried about the container bursting, but simply because any kind of accidental discharge could very well prove to be fatal.
If you ever have had the chance to try how it feels to be maced, you know you immediately end up in IFR without the possibility to read the gauges. The experience is blinding and very, very painful.

Comments are not for extended discussion; this conversation has been moved to chat.


text:
Angle of Attack influence on Adverse Yaw
we are analyzing an aircraft's lateral-directional control characteristics with self-made code. We have observed that the aileron's influence on the body axis yaw moment is quite sensitive to the angle of attack.
My question is if this makes any sense or if there is something wrong.
For Alpha = 0°

For Alpha = 5°

Edit: our model analyses the effects of the individual effects we simulated in CFD, relying on the superposition principle. For every component (including the ailerons) the following operations are made:
\begin{equation}
    \label{eq:decompforcas}
    \begin{bmatrix}
        F_x \\ F_y \\ F_z
    \end{bmatrix}
    =
    \begin{bmatrix}
        \cos{\alpha}\cos{\beta} & -\cos{\alpha}\sin{\beta} & -\sin{\alpha}\\
        \sin{\beta} & \cos{\beta} & 0\\
        \sin{\alpha}\cos{\beta} & -\sin{\alpha}\sin{\beta} & \cos{\alpha}
    \end{bmatrix}
    \begin{bmatrix}
        -qSC_D \\ qSC_Y \\ -qSC_L
    \end{bmatrix}
\end{equation}
\begin{equation}
    \label{eq:momentodocomponente}\begin{bmatrix}
        0 & l_z & -l_y\\
        l_z & 0 & -l_x\\
        -l_y & l_x & 0
    \end{bmatrix}
    \begin{bmatrix}
        F_x \\ F_y \\ F_z
    \end{bmatrix}
    +
    \begin{bmatrix}
        -qScC_l \\ qScC_m \\ -qScC_n
    \end{bmatrix}
    =
    \begin{bmatrix}
        M_x \\ M_y \\ M_z
    \end{bmatrix}
\end{equation}

You use the same reference length for all moments, which is uncommon. Normally, c$_m$ is multiplied with l$\mu$ and both c$_l$ and c$_n$ are multiplied with wingspan b or ½b. The adverse yaw over AoA looks wrong but without the code and the CFD results I cannot tell you why.

You're right about the reference length. I missed that when writing the equations, but it is done right practice, thanks!

That coukd very well be, with the up-going aileron in the “shade” of the wing’s upper surface. The adverse yaw effect from aileron hooking described in this answer mentions that the effect depends on angle-of-attack or camber of the wing.
At zero AoA and zero camber there would be no yawing moment due to aileron deflection.

Actually, our model is based on the superposition of the effects we simulated in CFD. As such, this doesn't include the aerodynamic effect you mention.
My guess is it is related to the aerodynamic-body axes transformations.

@rubemnobre Normally adverse yaw increases with angle of attack, since induced drag grows with AoA as well. The pyplot figures look wrong and a factor of more than 2 cannot be explained by an error in the transformation.

Good observation on how the transformation cannot explain a factor of more than 2.

Could it be that we only analyzed the aileron coefficients for a single angle of attack and assumed that it would vary the same in different angles?

Linear extrapolation of aerodynamic functions would not work.


text:
Approaching a towered airport for landing but no specific pattern entry direction from atc
Here’s the scenario that happened to me -
Flying into a busy class D, I contacted tower with the usual “XYZ tower, Warrior 12345 12 to the west at 2,500 full stop with Oscar”
They respond “Warrior 12345 remain at altitude of 2,500” giving me clearance into the airspace but no specific entry to the pattern like “enter left downwind, base, etc, for rwy 29”. (Pattern altitude is 1200AGL)
Never heard again from tower as I got within ~1-2 miles of airport and frequency was completely busy and couldn’t get a word in.
What would you do until they addressed you or you could get a word in? Fly in circles a couple miles west of the airport? Continue on current heading and fly over the airport above pattern?
Thanks

1200 AGL @757toga

Welcome to aviation.SE! What did you do in that situation? There are lots of variables at both towered and non-towered airports, and if you think that you didn't make the best decision at the time then adding more details might get a better response. This question is fairly similar and might help too.

@pondlife I did a 360 2 miles to the west of midfield at 2,500 and by the time I finished the 360 I was able to reach tower who then instructed me to drop into the down wind I was next to

Sounds like you did the most reasonable/safe thing you could. There's no (IMO) exact answer to this question, except the usual "you should aviate, navigate, communicate"